This application claims the benefit of Japanese Application No. Hei 11-362681 filed in Japan on Dec. 21, 1999.
The present invention relates to image display apparatus and, more particularly, to a head- or face-mounted image display apparatus that can be retained on the observer""s head or face.
Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-125791 is publicly known as a head- or face-mounted image display apparatus using a reflection type image display device, e.g. a reflection type liquid crystal display device (reflective LCD). The publication [JP(A) Hei 11-125791] proposes an optical system including a light source on the assumption that it uses a reflection type image display device. In the proposed optical system, however, no measure is taken to eliminate ghost light.
Let us explain ghost light by using a specific example of image display apparatus shown in FIG. 13. FIG. 13 is a diagram showing an optical path in an image display apparatus using two decentered prisms 10 and 20 and a reflective LCD 3. FIG. 13 shows the path of normal viewing light from a light source 5 to an eye E. In this example, the decentered prism 10 has four optical surfaces 11 to 14, and the decentered prism 20 has three optical surfaces 21 to 23. Light from the light source 5 enters the decentered prism 10 through a half-mirror surface provided on a surface 13 of the prism 10. Light rays refracted by the surface 13 are internally reflected by a surface 12 and then refracted by a surface 11 to illuminate the reflective LCD 3. Light reflected from the reflective LCD 3 enters the decentered prism 10 through the surface 11. The incident light is internally reflected successively by the surfaces 12 and 13 and exits the prism while being refracted through a surface 14. Then, the light enters the decentered prism 20 while being refracted through a surface 21. The incident light is totally reflected by a surface 23 and then internally reflected by a surface 22. The reflected light exits the prism 20 while being refracted through the surface 23 and reaches the eye E. Thus, the image displayed on the reflective LCD 3 is observed as an enlarged image.
However, as shown in FIG. 14, light from the light source 5 is partly reflected by the half-mirror surface on the surface 13 to reach the inner wall of a housing 6. Reflected light from the inner wall of the housing 6 does not travel along the path of normal viewing light shown in FIG. 13. For example, reflected light from the inner wall of the housing 6 passes through the prism 10 while being refracted through the surfaces 13 and 14 and enters the prism 20 while being refracted through the surface 21. The incident light is totally reflected by the surface 23 and then internally reflected by the surface 22. The reflected light exits the prism 20 while being refracted through the surface 23 and reaches the eye E. In this way, ghost light 7 occurs.
The present invention was made in view of the above-described problems associated with the prior art. An object of the present invention is to prevent the occurrence of ghost light in a viewing optical system using a reflection type image display device and a decentered prism. More specifically, in the viewing optical system, an illumination system and a viewing system share an optical path, and a half-mirror is used as a surface through which light from a light source enters the decentered prism. The reflection type image display device is illuminated with an approximately telecentric light beam. The present invention prevents the occurrence of ghost light which would otherwise be caused by light reflected from the half-mirror and further reflected from the inner wall of the housing of the apparatus or the like.
To attain the above-described object, the present invention provides an image display apparatus including a light source for illumination and a reflection type image display device adapted to display an image by reflecting a light beam from the light source. The image display apparatus further includes a viewing optical system for leading the image displayed by the reflection type image display device to an observer.
The viewing optical system has at least a first prism having at least a part thereof interposed in an optical path between the light source and the reflection type image display device.
The light source and the reflection type image display device are so positioned that an illuminating light beam from the light source is applied to the display surface of the reflection type image display device after passing through at least a part of the first prism.
The first prism has a first prism entrance surface through which an image display light beam reflected from the reflection type image display device enters the first prism. The first prism further has at least one first prism first reflecting surface for reflecting the light beam entering the first prism and a first prism exit surface through which the light beam entering the first prism exits. At least one of the first prism entrance surface, the first prism exit surface and the first prism first reflecting surface has a curved surface configuration that gives a power to a light beam.
A harmful light eliminating member is provided in an illuminating light path extending from the light source to the reflection type image display device in the vicinity of the light source to eliminate harmful light resulting from branching off a part of the light beam emitted from the light source from the illuminating light path when it passes through the first prism.
In this case, it is desirable that the viewing optical system should have at least a first prism that passes the illuminating light path between the light source and the reflection type image display device and a first optical element disposed on the observer side of the first prism apart from it. The first optical element should preferably have at least one first optical element first reflecting surface, which has a rotationally asymmetric curved surface configuration that gives a power to a light beam and corrects rotationally asymmetric aberrations due to decentration.
It is desirable that the first optical element should be a second prism having a second prism entrance surface through which the image light beam exiting the first prism enters the second prism. The second prism further has a second prism first reflecting surface for reflecting the light beam in the second prism and a second prism exit surface through which the light beam exits the second prism. It is desirable that at least one of the second prism entrance surface and the second prism exit surface should have a curved surface configuration that gives a power to a light beam.
The arrangement may be such that the first prism has at least one first prism second reflecting surface in the optical path between the first prism entrance surface and the first prism first reflecting surface. The first prism second reflecting surface has a rotationally asymmetric curved surface configuration that gives a power to a light beam and corrects rotationally asymmetric aberrations due to decentration.
The reasons for adopting the above-described arrangements in the present invention, together with the functions thereof, will be described below.
The image display apparatus according to the present invention includes a viewing optical system that has at least a first prism having at least a part thereof interposed in an optical path between a light source for illumination and a reflection type image display device. The light source and the reflection type image display device are so positioned that an illuminating light beam from the light source is applied to the display surface of the reflection type image display device after passing through at least a part of the first prism. In other words, a reflection type image display device, e.g. a reflective LCD, and a light source for illuminating it are disposed with at least a part of the first prism of the viewing optical system interposed therebetween so that the illuminating light beam illuminates the display surface of the reflection type image display device after passing through at least a part of the first prism.
The first prism has a first prism entrance surface through which an image display light beam reflected from the reflection type image display device enters the first prism. The first prism further has at least one first prism first reflecting surface for reflecting the light beam entering the first prism and a first prism exit surface through which the light beam entering the first prism exits. At least one of the first prism entrance surface, the first prism exit surface and the first prism first reflecting surface has a curved surface configuration that gives a power to a light beam. Accordingly, the observer can view the image displayed on the display surface of the reflection type image display device as an enlarged image.
According to the present invention, a harmful light eliminating member is provided in an illuminating light path extending from the light source to the reflection type image display device in the vicinity of the light source to eliminate harmful light resulting from branching off a part of the light beam emitted from the light source from the illuminating light path when it passes through the first prism. Accordingly, it is possible to prevent the occurrence of ghost light which would otherwise be caused by reflection of the harmful light from the inner wall of the housing of the apparatus or the like.
The viewing optical system in the present invention may have at least the above-described first prism and a first optical element disposed on the observer side of the first prism apart from it. The first optical element may have at least one first optical element first reflecting surface, which has a rotationally asymmetric curved surface configuration that gives a power to a light beam and corrects rotationally asymmetric aberrations due to decentration.
In other words, the first optical element disposed on the observer side of the first prism may be either a decentered reflecting mirror or a decentered prism. It is desirable that the reflecting surface (first reflecting surface) of the first optical element should have a rotationally asymmetric curved surface configuration that gives a power to a light beam and corrects rotationally asymmetric aberrations due to decentration.
Basically, the first prism, the first optical element and the second prism (described later) in the present invention are decentered optical systems. It is desirable that these decentered optical systems should be arranged so as to include at least one optical surface of positive power that is a rotationally asymmetric curved surface having no axis of rotational symmetry.
In a case where a decentered optical system is used, for example, as a viewing optical system of a head-mounted image display apparatus, it is necessary in order to eliminate a dead space and minimize the overall size of the apparatus to position an image display device and each optical surface constituting the decentered optical system so that the constituent elements are accommodated in the apparatus in as compact a form as possible. Consequently, the optical system must inevitably be decentered three-dimensionally, and this causes rotationally asymmetric aberration to occur. It is impossible to correct the rotationally asymmetric aberration by only a rotationally symmetric optical system. The best surface configuration for correcting the rotationally asymmetric aberration due to three-dimensional decentration is a rotationally asymmetric surface. Therefore, in the image display apparatus according to the present invention, it is desirable to use a rotationally asymmetric curved surface configuration that gives a power to a light beam and corrects rotationally asymmetric decentration aberrations as the configuration of at least the reflecting surface of the first optical element.
A free-form surface used in the present invention as a rotationally asymmetric surface having no axis of rotational symmetry is defined by the following equation. The Z-axis of the defining equation is the axis of a free-form surface.                     Z        =                                            cr              2                        /                          [                              1                +                                                      {                                          1                      -                                                                        (                                                      1                            +                            k                                                    )                                                ⁢                                                  c                          2                                                ⁢                                                  r                          2                                                                                      }                                                              ]                                +                                    ∑                              j                =                2                            66                        ⁢                          xe2x80x83                        ⁢                                          C                j                            ⁢                              X                m                            ⁢                              Y                n                                                                        (        a        )            
In the equation (a), the first term is a spherical surface term, and the second term is a free-form surface term.
In the spherical surface term:
c: the curvature at the vertex
k: a conic constant   r  =                    xe2x80x83            ⁢              (                              X            2                    +                      Y            2                          )            
The free-form surface term is given by                                           ∑                          j              =              2                        66                    ⁢                      xe2x80x83                    ⁢                                    C              j                        ⁢                          X              m                        ⁢                          Y              n                                      =                  xe2x80x83                ⁢                                            C              2                        ⁢            X                    +                                    C              3                        ⁢            Y                    +                                                  xe2x80x83                ⁢                                            C              4                        ⁢                          X              2                                +                                    C              5                        ⁢            XY                    +                                    C              6                        ⁢                          Y              2                                +                                                  xe2x80x83                ⁢                                            C              7                        ⁢                          X              3                                +                                    C              8                        ⁢                          X              2                        ⁢            Y                    +                                    C              9                        ⁢                          XY              2                                +                                    C              10                        ⁢                          Y              3                                +                                                  xe2x80x83                ⁢                                            C              11                        ⁢                          X              4                                +                                    C              12                        ⁢                          X              3                        ⁢            Y                    +                                    C              13                        ⁢                          X              2                        ⁢                          Y              2                                +                                    C              14                        ⁢                          XY              3                                +                                    C              15                        ⁢                          Y              4                                +                                                  xe2x80x83                ⁢                                            C              16                        ⁢                          X              5                                +                                    C              17                        ⁢                          X              4                        ⁢            Y                    +                                    C              18                        ⁢                          X              3                        ⁢                          Y              2                                +                                    C              19                        ⁢                          X              2                        ⁢                          Y              3                                +                                    C              20                        ⁢                          XY              4                                +                                                  xe2x80x83                ⁢                                            C              21                        ⁢                          Y              5                                +                                                  xe2x80x83                ⁢                                            C              22                        ⁢                          X              6                                +                                    C              23                        ⁢                          X              5                        ⁢            Y                    +                                    C              24                        ⁢                          X              4                        ⁢                          Y              2                                +                                    C              25                        ⁢                          X              3                        ⁢                          Y              3                                +                                    C              26                        ⁢                          X              2                        ⁢                          Y              4                                +                                                  xe2x80x83                ⁢                                            C              27                        ⁢                          XY              5                                +                                    C              28                        ⁢                          Y              6                                +                                                  xe2x80x83                ⁢                                            C              29                        ⁢                          X              7                                +                                    C              30                        ⁢                          X              6                        ⁢            Y                    +                                    C              31                        ⁢                          X              5                        ⁢                          Y              2                                +                                    C              32                        ⁢                          X              4                        ⁢                          Y              3                                +                                    C              33                        ⁢                          X              3                        ⁢                          Y              4                                +                                                  xe2x80x83                ⁢                                            +                              C                34                                      ⁢                          X              2                        ⁢                          Y              5                                +                                    C              35                        ⁢                          XY              6                                +                                    C              36                        ⁢                          Y              7                        ⁢                          xe2x80x83                        ⁢            ⃛                              
where Cj (j is an integer of 2 or higher) are coefficients.
In general, the above-described free-form surface does not have planes of symmetry in both the XZ- and YZ-planes. However, a free-form surface having only one plane of symmetry parallel to the YZ-plane is obtained by making all terms of odd-numbered degrees with respect to X zero. A free-form surface having only one plane of symmetry parallel to the XZ-plane is obtained by making all terms of odd-numbered degrees with respect to Y zero. Accordingly, at least one of free-form surfaces used in the present invention should have a surface configuration in which at least one of the terms of odd-numbered degrees with respect to X is not zero and at least one of the terms of odd-numbered degrees with respect to Y is not zero.
In addition, free-form surfaces as the above-described rotationally asymmetric surfaces having no axis of rotational symmetry may be defined by Zernike polynomials. That is, the configuration of a free-form surface may be defined by the following equation (b). The Z-axis of the defining equation (b) is the axis of Zernike polynomial. A rotationally asymmetric surface is defined by polar coordinates of the height of the Z-axis with respect to the XY-plane. In the equation (b), R is the distance from the Z-axis in the XY-plane, and A is the azimuth angle about the Z-axis, which is expressed by the angle of rotation measured from the X-axis.                                                         x              =                              xe2x80x83                            ⁢                              R                xc3x97                                  cos                  ⁡                                      (                    A                    )                                                                                                                          y              =                              xe2x80x83                            ⁢                              R                xc3x97                sin                ⁢                                  xe2x80x83                                ⁢                                  (                  A                  )                                                                                                        z              =                              xe2x80x83                            ⁢                                                D                  2                                +                                                                                                        xe2x80x83                            ⁢                                                                    D                    3                                    ⁢                  R                  ⁢                                      xe2x80x83                                    ⁢                                      cos                    ⁡                                          (                      A                      )                                                                      +                                                      D                    4                                    ⁢                  R                  ⁢                                      xe2x80x83                                    ⁢                                      sin                    ⁡                                          (                      A                      )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                                    D                    5                                    ⁢                                      R                    2                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          2                      ⁢                      A                                        )                                                  +                                                      D                    6                                    ⁡                                      (                                                                  R                        2                                            -                      1                                        )                                                  +                                                      D                    7                                    ⁢                                      R                    2                                    ⁢                                      sin                    ⁡                                          (                                              2                        ⁢                        A                                            )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                                    D                    8                                    ⁢                                      R                    3                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          3                      ⁢                      A                                        )                                                  +                                                                            D                      9                                        ⁡                                          (                                                                        3                          ⁢                                                      R                            3                                                                          -                                                  2                          ⁢                          R                                                                    )                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                    A                    )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      10                                        ⁡                                          (                                                                        3                          ⁢                                                      R                            3                                                                          -                                                  2                          ⁢                          R                                                                    )                                                        ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                    A                    )                                                  +                                                      D                    11                                    ⁢                                      R                    3                                    ⁢                                      sin                    ⁡                                          (                                              3                        ⁢                        A                                            )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                                    D                    12                                    ⁢                                      R                    4                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          4                      ⁢                      A                                        )                                                  +                                                                            D                      13                                        ⁡                                          (                                                                        4                          ⁢                                                      R                            4                                                                          -                                                  3                          ⁢                                                      R                            2                                                                                              )                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          2                      ⁢                      A                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                    D                    14                                    ⁡                                      (                                                                  6                        ⁢                                                  R                          4                                                                    -                                              6                        ⁢                                                  R                          2                                                                    +                      1                                        )                                                  +                                                                            D                      15                                        ⁡                                          (                                                                        4                          ⁢                                                      R                            4                                                                          -                                                  3                          ⁢                                                      R                            2                                                                                              )                                                        ⁢                                      sin                    ⁡                                          (                                              2                        ⁢                        A                                            )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                                    D                    16                                    ⁢                                      R                    4                                    ⁢                                      sin                    ⁡                                          (                                              4                        ⁢                        A                                            )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                                    D                    17                                    ⁢                                      R                    5                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          5                      ⁢                      A                                        )                                                  +                                                                            D                      18                                        ⁡                                          (                                                                        5                          ⁢                                                      R                            5                                                                          -                                                  4                          ⁢                                                      R                            3                                                                                              )                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          3                      ⁢                      A                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      19                                        ⁡                                          (                                                                        10                          ⁢                                                      R                            5                                                                          -                                                  12                          ⁢                                                      R                            3                                                                          +                                                  3                          ⁢                          R                                                                    )                                                        ⁢                                      cos                    ⁡                                          (                      A                      )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      20                                        ⁡                                          (                                                                        10                          ⁢                                                      R                            5                                                                          -                                                  12                          ⁢                                                      R                            3                                                                          +                                                  3                          ⁢                          R                                                                    )                                                        ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                    A                    )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      21                                        ⁡                                          (                                                                        5                          ⁢                                                      R                            5                                                                          -                                                  4                          ⁢                                                      R                            3                                                                                              )                                                        ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          3                      ⁢                      A                                        )                                                  +                                                      D                    22                                    ⁢                                      R                    5                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          5                      ⁢                      A                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                    D                    23                                    ⁢                                      R                    6                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          6                      ⁢                      A                                        )                                                  +                                                                            D                      24                                        ⁡                                          (                                                                        6                          ⁢                                                      R                            6                                                                          -                                                  5                          ⁢                                                      R                            4                                                                                              )                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          4                      ⁢                      A                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      25                                        ⁡                                          (                                                                        15                          ⁢                                                      R                            6                                                                          -                                                  20                          ⁢                                                      R                            4                                                                          +                                                  6                          ⁢                                                      R                            2                                                                                              )                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          2                      ⁢                      A                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                    D                    26                                    ⁡                                      (                                                                  20                        ⁢                                                  R                          6                                                                    -                                              30                        ⁢                                                  R                          4                                                                    +                                              12                        ⁢                                                  R                          2                                                                    -                      1                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      27                                        ⁡                                          (                                                                        15                          ⁢                                                      R                            6                                                                          -                                                  20                          ⁢                                                      R                            4                                                                          +                                                  6                          ⁢                                                      R                            2                                                                                              )                                                        ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          2                      ⁢                      A                                        )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          D                      28                                        ⁡                                          (                                                                        6                          ⁢                                                      R                            6                                                                          -                                                  5                          ⁢                                                      R                            4                                                                                              )                                                        ⁢                                      sin                    ⁡                                          (                                              4                        ⁢                        A                                            )                                                                      +                                                      D                    29                                    ⁢                                      R                    6                                    ⁢                                      sin                    ⁡                                          (                                              6                        ⁢                        A                                            )                                                        ⁢                                      xe2x80x83                                    ⁢                  …                                                                                        (        b        )            
where Dm (m is an integer of 2 or higher) are coefficients.
It should be noted that to design an optical system symmetric with respect to the X-axis direction, D4, D5, D6, D10, D11, D12, D13, D14, D20, D21, D22 . . . should be used.
The above defining equations are shown to exemplify a rotationally asymmetric surface having no axis of rotational symmetry. Therefore, the same advantageous effect can be obtained for any other defining equation that expresses such a rotationally asymmetric surface.
In the present invention, the first optical element may be either a decentered reflecting mirror or a decentered prism, as stated above. When a decentered prism is used as the first optical element, the first optical element may be formed, for example, from a second prism having a second prism entrance surface through which the image light beam exiting the first prism enters the second prism. The second prism further has a second prism first reflecting surface for reflecting the light beam in the second prism and a second prism exit surface through which the light beam exits the second prism. In this case, it is desirable that at least one of the second prism entrance surface and the second prism exit surface should have a curved surface configuration that gives a power to a light beam.
The arrangement may be such that the first prism has at least one first prism second reflecting surface in the optical path between the first prism entrance surface and the first prism first reflecting surface. The first prism second reflecting surface has a rotationally asymmetric curved surface configuration that gives a power to a light beam and corrects rotationally asymmetric aberrations due to decentration. This arrangement also allows the observer to view the image displayed on the display surface of the reflection type image display device as an enlarged image while correcting decentration aberrations due to the decentered arrangement.
Incidentally, the harmful light eliminating member for eliminating harmful light resulting from branching off a part of the illuminating light beam from the illuminating light path when it passes through the first prism should preferably be provided outside the optical path of the viewing optical system and arranged to eliminate harmful light branched off from the first prism so that the harmful light will not return to enter the viewing optical system.
A specific example of the arrangement of the harmful light eliminating member is as follows. The first prism first reflecting surface of the first prism is arranged to have both transmitting and reflecting actions so that the first prism first reflecting surface reflects light in the viewing light path for leading the image light beam from the reflection type image display device to the observer and transmits light in the illuminating light path extending from the light source to the reflection type image display device to allow the illuminating light beam to enter the first prism. The harmful light eliminating member is arranged to eliminate a light beam reflected to the outside of the first prism as a harmful light beam when the illuminating light beam from the light source enters the first prism through the first prism first reflecting surface.
In this case, the harmful light eliminating member may be a wall surface that is slanted so that when a part of the illuminating light beam is reflected from the first prism first reflecting surface to the outside of the first prism as a harmful light beam, the slanted wall surface reflects the harmful light beam in a direction from which the harmful light beam cannot enter the viewing optical system.
The harmful light eliminating member may be a member having light-shielding lines cut therein to cause irregular reflection so as to reduce the intensity of a part of the illuminating light beam reflected from the first prism first reflecting surface to the outside of the first prism as a harmful light beam.
The harmful light eliminating member may be a combination of a first polarizing plate provided between the light source and the first prism first reflecting surface and a polarization beam splitter formed on the first prism first reflecting surface. In this case, the harmful light eliminating member may be arranged such that the illuminating light beam becomes either a p- or s- polarized light beam by passing through the first polarizing plate, and the polarization beam splitter has a polarizing action to pass the polarized light beam passing through the first polarizing plate, thereby preventing the illuminating light beam from being reflected from the first prism first reflecting surface to the outside of the first prism as a harmful light beam.
It is also possible to prevent harmful light from returning to enter the viewing optical system by adopting the following arrangement. The first prism first reflecting surface of the first prism is formed from a half-mirror surface. The harmful light eliminating member is formed from a combination of a first polarizing plate provided between the light source and the first prism first reflecting surface of the first prism and a second polarizing plate placed at a position where it receives a light beam that is reflected by the half-mirror action of the first prism first reflecting surface to the outside of the first prism after passing through the first polarizing plate. The second polarizing plate is disposed in crossed Nicol relation to the first polarizing plate.
The harmful light eliminating member may be a light-absorbing member placed at a position where it receives a part of the illuminating light beam reflected from the first prism first reflecting surface to the outside of the first prism.
When the viewing optical system of the image display apparatus according to the present invention is formed from the first prism and the second prism placed on the observer side of the first prism, it is desirable that the second prism first reflecting surface should be a rotationally asymmetric free-form surface having only one plane of symmetry.
When the viewing optical system of the image display apparatus according to the present invention is formed from the first prism having a first reflecting surface and a second reflecting surface, it is desirable that the first prism second reflecting surface should be a rotationally asymmetric free-form surface having only one plane of symmetry.
It is desirable that a light beam from the light source forms at least an illuminating light path in which the light beam enters the first prism through the first prism first reflecting surface and an illuminating light beam is applied to the reflection type image display device through the first prism entrance surface, and that a light beam from an image displayed on the reflection type image display device forms at least a viewing light path in which the light beam enters the first prism through the first prism entrance surface and is reflected by the first prism first reflecting surface, and further that the illuminating light path and the viewing light path should form approximately the identical optical path (the widths of the two light beams are not equal to each other) in which the directions of travel of the two light beams are reverse to each other. With this arrangement, the illuminating light beam can be applied approximately normal to the reflection type image display device.
When the viewing optical system of the image display apparatus according to the present invention is formed from the first prism and the second prism placed on the observer side of the first prism, it is desirable that the first prism has a first prism second reflecting surface in the optical path between the first prism entrance surface and the first prism first reflecting surface, and the first prism second reflecting surface should be a rotationally asymmetric free-form surface having only one plane of symmetry.
In this case, the first prism may be arranged such that an optical path connecting together the first prism entrance surface and the first prism second reflecting surface and an optical path connecting together the first prism first reflecting surface and the first prism exit surface intersect each other in the first prism.
The first prism having the above-described configuration enables an increase in the degree of freedom for aberration correction and produces minimal aberrations. In addition, because the two reflecting surfaces in the first prism can be positioned with a high degree of symmetry, aberrations produced by the two reflecting surfaces are corrected with these reflecting surfaces by canceling the aberrations each other. Therefore, the amount of aberration produced in the first prism is favorably small. Furthermore, because the above-described two optical paths are arranged to intersect each other in the first prism, the optical path length can be made long in comparison to a prism structure in which the optical path is simply folded. Accordingly, the prism can be made compact in size, considering its optical path length. In addition, because it is possible to lengthen the overall distance and to set a relatively weak power for each reflecting surface, the amount of aberration produced in the first prism is minimized.
In this case, it is desirable that the second prism should have a second prism second reflecting surface in the viewing light path between the second prism entrance surface and the second prism first reflecting surface, and that the second prism second reflecting surface should have a curved surface configuration that gives a power to a light beam.
The arrangement may be such that the second prism second reflecting surface and the second prism exit surface are the identical surface serving as both reflecting and transmitting surfaces. With this arrangement, the light beam entering the second prism through the second prism entrance surface is totally reflected by the second prism exit surface and then reflected by the second prism first reflecting surface to exit the second prism through the second prism exit surface.
If the second prism is of the type in which the second reflecting surface and the exit surface are formed from a single surface serving as both reflecting and transmitting surfaces, the second reflecting surface reflects incident rays to the first reflecting surface at a large angle of deviation, and the first reflecting surface bends the rays at a minimal angle of reflection. Therefore, it is possible to reduce the thickness of the prism in the direction of the emergent rays.
When the first prism is formed from a decentered prism having a first reflecting surface and a second reflecting surface, it is desirable that the first prism should have at least a first prism second reflecting surface in the optical path between the first prism entrance surface and the first prism first reflecting surface, and the first prism second reflecting surface should be a rotationally asymmetric free-form surface having only one plane of symmetry.
In this case also, the first prism may be arranged such that an optical path connecting together the first prism entrance surface and the first prism second reflecting surface and an optical path along which the light beam reflected from the first prism first reflecting surface travels to reach the subsequent optical functional surface intersect each other in the first prism.
The first prism having the above-described configuration enables an increase in the degree of freedom for aberration correction and produces minimal aberrations, as stated above. In addition, because the two reflecting surfaces in the first prism can be positioned with a high degree of symmetry, aberrations produced by the two reflecting surfaces are corrected with these reflecting surfaces by canceling the aberrations each other. Therefore, the amount of aberration produced in the first prism is favorably small. Furthermore, because the above-described two optical paths are arranged to intersect each other in the first prism, the optical path length can be made long in comparison to a prism structure in which the optical path is simply folded. Accordingly, the prism can be made compact in size, considering its optical path length. In addition, because it is possible to lengthen the overall distance and to set a relatively weak power for each reflecting surface, the amount of aberration produced in the first prism is minimized.
It is also desirable that the first prism should have a first prism third reflecting surface and a first prism fourth reflecting surface in the viewing light path between the first prism first reflecting surface and the first prism exit surface, and at least either one of the first prism third reflecting surface and the first prism fourth reflecting surface should be a rotationally asymmetric free-form surface having only one plane of symmetry.
In this case, the arrangement may be such that the first prism third reflecting surface and the first prism exit surface are the identical surface serving as both reflecting and transmitting surfaces. With this arrangement, the light beam reflected from the first prism first reflecting surface is totally reflected by the first prism exit surface (first prism third reflecting surface) and then reflected by the first prism fourth reflecting surface to exit the first prism through the first prism exit surface.
It should be noted that the present invention includes a head-mounted image display apparatus having an apparatus body unit incorporating the light source for illumination, the reflection type image display device, the viewing optical system and the harmful light eliminating member in the above-described image display apparatus. The head-mounted image display apparatus further has a support member for supporting the apparatus body unit in such a manner that the apparatus body unit is fitted to the observer""s face. In this case, the present invention includes a head-mounted image display apparatus for both eyes that has an apparatus body unit incorporating a light source for illumination, a reflection type image display device for the right eye, a viewing optical system for the right eye, a reflection type image display device for the left eye, a viewing optical system for the left eye, and a harmful light eliminating member. The head-mounted image display apparatus for both eyes further has a support member for supporting the apparatus body unit in such a manner that the apparatus body unit is fitted to the observer""s face.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.